In computer controlled automated mechanical drilling processes it is necessary to insure that the drill bit remains sharp and unbroken throughout the drilling operation. This is especially important in the drilling of printed circuit boards. In the manufacturing of single and multilayer circuit boards, holes are drilled through the board to accommodate the electrical connections required for the circuits. In many applications, the number of holes drilled can range from 4,000 to 10,000 holes. Generally, for drilling a large number of holes in the circuit boards, a drilling machine is used having multiple spindles. The large volume of holes being drilled requires that the drill bits be of a very small diameter. Currently, the drill bit being used is as small as 16 mil.
The use of these very fine drill bits causes the bits to become blunt or broken very easily during the drilling operation. Since the machines are automatically controlled, the drilling operation cannot be stopped and a broken or blunt drill will continue to spin and damage the boards. The holes drilled with a blunt or broken drill bit will be defective. This is not acceptable in the manufacture of circuit boards where accuracy is required. The defective holes must be reworked at a significant delay and expense. Frequently, proper reworking cannot be accomplished and an entire set of boards must be rejected.
Furthermore, in the manufacturing of multilayer circuit boards, it is often necessary to drill only to certain intermediate layers for the connections of the electrical circuits. However, heretofore there has not been available an efficient and accurate system for drilling multilayer boards to a predetermined depth. Thus, manufacturers are forced to drill through all the layers. In one approach to the problem, the machine is controlled by an operator to drill for a preselected time at a predetermined speed. This practice has been found to be unsuccessful because the drilling speeds vary as the drill bit progresses through inhomogeneous mediums. In addition, the operators must know precisely when the drilling commences and what the drilling speed is. Hence, the machine drills through more layers than is needed requiring additional electrical connections and unnecessary costs.